Multiple-layer printed circuit boards or printed wiring boards (PWBs) are used for mounting integrated circuits (ICs) and other components. The push to decrease circuit size and weight and to operate at higher frequencies and clock speeds has led to smaller components generating greater heat and being placed more closely together on the PWB. Additional size and speed improvements have also been achieved by reducing the footprints of the components by using leadless chip carriers.
The greater density of components on the PWBs and hotter components resulted in thermal management problems. The Coefficient of Thermal Expansion (“CTE”) mismatch between the PWBs and the components becomes more important when greater temperatures are generated. CTE mismatch between the PWBs and components can result in fracture or fatigue during the thermal cycling caused by powering on and off of electronic devices. Leadless chip carriers are especially susceptible to disengagement from the PWB when there are CTE mismatches. Solder joints and connections tend to pull apart in the “tug-of-war” introduced by the CTE mismatch.
Prior PWB designs have used metal constraining layers or cores, such as copper-invar-copper, aluminum or steel, to lower the board's CTE. However, these materials add undesirable weight. U.S. Pat. No. 4,318,954 to Jensen provides an example of a PWB design for use in cycling thermal environments that uses lightweight carbon based constraining layers to lower the board's CTE. U.S. Pat. No. 4,591,659 to Leibowitz also demonstrates that carbon constraining layers can serve as thermal conductors for carrying heat away from the components mounted on the PWB in addition to lowering the board's CTE. U.S. Pat. No. 4,318,954 to Jensen and U.S. Pat. No. 4,591,659 to Leibowitz are incorporated by reference in their entirety to the present disclosure.
The ability of previous PWBs to conduct heat away from the components mounted on their surfaces is limited by the prepreg used to prevent electrical conductivity between the functional layers of the PWB. The materials used in prepreg have poor thermal conduction properties. Therefore, the ability of the carbon constraining layer to conduct heat away from the surface of the board was limited by the amount of prepreg between it and the surface of the board. The carbon material used in the carbon constraining layers is electrically conductive, which required the functional layers of the PWB in prior structures to be electrically insulated from the carbon constraining layers in order to prevent short circuits and cross talk. In previous designs, this requirement places a lower limit on the distance between the carbon constraining layers and the surface of the board equivalent to the minimum amount of prepreg required to insulate the functional layers of the board from each other and from the carbon constraining layers. This lower limit translated into an upper limit on the amount of heat that could be conducted away from the surface of the PWB. Accordingly there was a need for a PWB that possessed mechanical strength with a low CTE and that exceeds the upper limit on the amount of heat that can be conducted away from the surface of the PWB which was inherent in previous designs.